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Law enforcement officials are investigating after someone left an explosive device at a Home Depot on Long Island, and they are concerned the person may try to do it again, NBC 4 New York has learned.

The crude pipe bomb was left at the store on New York Avenue in Huntington as part of an extortion attempt, law enforcement sources said. A letter was also mailed there demanding money and describing where the bomb was inside the building.

The letter had been sitting for some time in the store before it was opened Monday. It stated that if the money wasn't paid by Black Friday in November, more devices could be planted. Sources said the threat letter was mailed from Long Island.

Responding police located the pipe bomb on a shelf behind some boxes.

Customer Peter Howard said he went to the store Monday to return an item and was told he couldn't enter because of a fire drill.

"When I came back at 9:30 again, thinking the fire drill would be over, there was police tape across here and emergency vehicles over there," he said.

The pipe bomb was destroyed by the Suffolk County Bomb Squad Monday evening.

Sources said the bomb did not have a timer attached to it but if someone had opened the box containing the device, it could have detonated.

The FBI has joined Suffolk County Police in the investigation. They're working to find the suspect in case he or she tries to place other devices at other stores.

Police across Long Island and in New York City have been made aware of the threat, as have Home Depot stores in the area, the sources said.

“We are cooperating with authorities in the investigation,” said Stephen Holmes, a Home Depot spokesman. "The safety of our customers and our employees is of the utmost importance to us."

Suffolk police said there were about 50 people in the store when it was shut down Monday.